feat(filter): add higher-order lettable version of filter

src/operator/filter.ts

@@ -1,7 +1,6 @@
-import { Operator } from '../Operator';
-import { Subscriber } from '../Subscriber';
+
 import { Observable } from '../Observable';
-import { TeardownLogic } from '../Subscription';
+import { filter as higherOrderFilter } from '../operators';
 
 /* tslint:disable:max-line-length */
 export function filter<T, S extends T>(this: Observable<T>,
@@ -53,47 +52,5 @@ export function filter<T>(this: Observable<T>,
  */
 export function filter<T>(this: Observable<T>, predicate: (value: T, index: number) => boolean,
                           thisArg?: any): Observable<T> {
-  return this.lift(new FilterOperator(predicate, thisArg));
-}
-
-class FilterOperator<T> implements Operator<T, T> {
-  constructor(private predicate: (value: T, index: number) => boolean,
-              private thisArg?: any) {
-  }
-
-  call(subscriber: Subscriber<T>, source: any): TeardownLogic {
-    return source.subscribe(new FilterSubscriber(subscriber, this.predicate, this.thisArg));
-  }
-}
-
-/**
- * We need this JSDoc comment for affecting ESDoc.
- * @ignore
- * @extends {Ignored}
- */
-class FilterSubscriber<T> extends Subscriber<T> {
-
-  count: number = 0;
-
-  constructor(destination: Subscriber<T>,
-              private predicate: (value: T, index: number) => boolean,
-              private thisArg: any) {
-    super(destination);
-    this.predicate = predicate;
-  }
-
-  // the try catch block below is left specifically for
-  // optimization and perf reasons. a tryCatcher is not necessary here.
-  protected _next(value: T) {
-    let result: any;
-    try {
-      result = this.predicate.call(this.thisArg, value, this.count++);
-    } catch (err) {
-      this.destination.error(err);
-      return;
-    }
-    if (result) {
-      this.destination.next(value);
-    }
-  }
+  return higherOrderFilter(predicate, thisArg)(this);
 }

src/operators/filter.ts

@@ -0,0 +1,100 @@
+import { Operator } from '../Operator';
+import { Subscriber } from '../Subscriber';
+import { Observable } from '../Observable';
+import { TeardownLogic } from '../Subscription';
+import { OperatorFunction } from './OperatorFunction';
+
+/* tslint:disable:max-line-length */
+export function filter<T, S extends T>(predicate: (value: T, index: number) => value is S,
+                                       thisArg?: any): OperatorFunction<T, S>;
+export function filter<T>(predicate: (value: T, index: number) => boolean,
+                          thisArg?: any): OperatorFunction<T, T>;
+/* tslint:enable:max-line-length */
+
+/**
+ * Filter items emitted by the source Observable by only emitting those that
+ * satisfy a specified predicate.
+ *
+ * <span class="informal">Like
+ * [Array.prototype.filter()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter),
+ * it only emits a value from the source if it passes a criterion function.</span>
+ *
+ * <img src="./img/filter.png" width="100%">
+ *
+ * Similar to the well-known `Array.prototype.filter` method, this operator
+ * takes values from the source Observable, passes them through a `predicate`
+ * function and only emits those values that yielded `true`.
+ *
+ * @example <caption>Emit only click events whose target was a DIV element</caption>
+ * var clicks = Rx.Observable.fromEvent(document, 'click');
+ * var clicksOnDivs = clicks.filter(ev => ev.target.tagName === 'DIV');
+ * clicksOnDivs.subscribe(x => console.log(x));
+ *
+ * @see {@link distinct}
+ * @see {@link distinctUntilChanged}
+ * @see {@link distinctUntilKeyChanged}
+ * @see {@link ignoreElements}
+ * @see {@link partition}
+ * @see {@link skip}
+ *
+ * @param {function(value: T, index: number): boolean} predicate A function that
+ * evaluates each value emitted by the source Observable. If it returns `true`,
+ * the value is emitted, if `false` the value is not passed to the output
+ * Observable. The `index` parameter is the number `i` for the i-th source
+ * emission that has happened since the subscription, starting from the number
+ * `0`.
+ * @param {any} [thisArg] An optional argument to determine the value of `this`
+ * in the `predicate` function.
+ * @return {Observable} An Observable of values from the source that were
+ * allowed by the `predicate` function.
+ * @method filter
+ * @owner Observable
+ */
+export function filter<T>(predicate: (value: T, index: number) => boolean,
+                          thisArg?: any): OperatorFunction<T, T> {
+  return function filterOperatorFunction(source: Observable<T>): Observable<T> {
+    return source.lift(new FilterOperator(predicate, thisArg));
+  };
+}
+
+class FilterOperator<T> implements Operator<T, T> {
+  constructor(private predicate: (value: T, index: number) => boolean,
+              private thisArg?: any) {
+  }
+
+  call(subscriber: Subscriber<T>, source: any): TeardownLogic {
+    return source.subscribe(new FilterSubscriber(subscriber, this.predicate, this.thisArg));
+  }
+}
+
+/**
+ * We need this JSDoc comment for affecting ESDoc.
+ * @ignore
+ * @extends {Ignored}
+ */
+class FilterSubscriber<T> extends Subscriber<T> {
+
+  count: number = 0;
+
+  constructor(destination: Subscriber<T>,
+              private predicate: (value: T, index: number) => boolean,
+              private thisArg: any) {
+    super(destination);
+    this.predicate = predicate;
+  }
+
+  // the try catch block below is left specifically for
+  // optimization and perf reasons. a tryCatcher is not necessary here.
+  protected _next(value: T) {
+    let result: any;
+    try {
+      result = this.predicate.call(this.thisArg, value, this.count++);
+    } catch (err) {
+      this.destination.error(err);
+      return;
+    }
+    if (result) {
+      this.destination.next(value);
+    }
+  }
+}

src/operators/index.ts

@@ -1,3 +1,5 @@
 export { OperatorFunction } from './OperatorFunction';
+export { filter } from './filter';
+
 export { map } from './map';
 export { mergeMap } from './mergeMap';